What Does Your Immune System Have to Do with Your Behavior?

Core Concepts

Sickness behavior as a motivational state

The behavior of a sick animal looks passive, but it is actively organized by the brain, much like hunger or thirst. The pioneering ethological view (often credited to Benjamin Hart, 1988) framed sickness behavior as a motivated state that reorders priorities: instead of foraging, mating, or exploring, the animal conserves energy and avoids further exposure. Key components:

• Lethargy and reduced activity — energy is redirected to the immune response and to raising body temperature.
• Anorexia — reduced food intake during acute infection.
• Sleep changes — increased and deeper sleep (sleep itself supports immune function).
• Social withdrawal — sick individuals disengage from social contact, reducing transmission.
• Anhedonia and reduced grooming — loss of interest in normally rewarding activities.

These behaviors are triggered by the same cytokines in a dose-dependent way, and they can be induced in animals by injecting cytokines (or LPS, which stimulates their release) — evidence that the immune signal itself is the cause.

Fever: the brain's thermostat is reset

Fever is a hallmark of infection, and it is a brain response. Proinflammatory cytokines act on the preoptic area of the hypothalamus — the same region that regulates body temperature (see Chapter 16, Topic 3). Through mechanisms including prostaglandin E2 (PGE2) synthesis, they raise the set point of the thermostat: the body then actively generates heat (shivering, vasoconstriction) to reach the new target. Moderate fever is commonly taught to inhibit some pathogens and enhance immune cell function, though the optimality and clinical management of fever remain nuanced and context-dependent.

Cytokines change neurotransmitter and circuit function

Cytokines reaching the brain alter the activity of neural circuits, not just the hypothalamus. Research (largely in animal models) shows cytokine-induced changes in:

• Monoamine systems — altered serotonin and norepinephrine turnover, implicated in the fatigue and mood changes of sickness.
• Hypothalamic–pituitary–adrenal (HPA) axis — cytokines activate CRH neurons in the paraventricular nucleus, raising cortisol; this both supports metabolism during illness and restrains inflammation (a feedback loop with Topic 3).
• Motivational circuitry — reduced sensitivity to rewards (anhedonia), consistent with the overlap between sickness behavior and depression.

These are active research areas, and the mapping from cytokine to specific symptom is far from one-to-one.

The sickness–depression overlap

Because chronic inflammation is associated with depressive symptoms — fatigue, sleep disturbance, appetite change, anhedonia, social withdrawal — researchers study whether inflammation contributes to major depression in some people. Observational and experimental findings support an association (e.g., elevated inflammatory markers in some individuals with depression; depressive symptoms after cytokine therapy such as interferon treatment), but this does not mean depression is "just" inflammation: depression is heterogeneous, and causality, direction, and individual susceptibility remain under active investigation. This is a good example of the requirement to separate association from causation.

Individual differences and modulation

Not everyone responds to an infection with identical sickness behavior. Age, sex, prior infection history, stress history, and genetic background modulate the intensity of the response. Chronic stress, for example, can alter inflammatory signaling and the behavioral response to immune challenge — linking this topic to Chapter 12 on stress.

Common Confusions